This invention pertains to making flat hems or modified flat hems, sometimes called pinch hems, for aluminum panels. More specifically this invention pertains to a method of making flat, sharp pinch hems on aluminum automotive body panels that have been formed in a superplastic or other forming operation that involves stretching of an aluminum sheet.
In a continuing effort to reduce weight in automotive vehicles, aluminum alloys are substituted for steels in many applications. Aluminum sheet alloys are not as easy to form or hem or weld, as are low carbon steel sheets. Considerable effort has been expended to develop aluminum alloys for sheet metal forming and welding. For example, U.S. Pat. No. 6,253,588, Rashid, et al, entitled xe2x80x9cQuick Plastic Forming of Aluminum Alloy Sheet Metalxe2x80x9d describes methods for stretch forming large sheets of superplastically formable (SPF) aluminum alloys into automotive body panels. Cold rolled Aluminum Alloy 5083 sheet that has been recrystallized to a very fine grain structure, sometimes called a pseudo-single phase material, is an example of a suitable SPF alloy.
Automobile body panels are usually of stylish threedimensional curvature and require a commercial quality outer surface for painting or other finishing. The methods of the ""588 patent have been used by the assignee of this invention to make inner and outer deck lid panels and inner and outer lift gate panels. Other candidate vehicle closure panels include door and hood panels. Automobile closure panels have to be formed with commercially acceptable appearance and with suitable dimensional accuracy for fitting with adjacent body structures. Furthermore, flange portions of the outer panel must be capable of bending around the edges of an assembled inner panel in a hem that secures the panels in a rattle-free and attractive bond.
Commercial aluminum alloy sheet material (such as alloys of the 5xxx and 6xxx series) for body panel stamping processes are difficult to hem. Stamped sheets of these alloys often require a rope hem in which the flange of the outer panel is bent in a broad loop, as though folded around the circumference of a rope thicker than the inner sheet, to engage the inner panel. Such open hems have been necessary with aluminum sheet alloys if cracking or fracture of the hemmed material is to be avoided. Certain SPF aluminum panels stretch formed at about 400xc2x0 C. to 500xc2x0 C. as per the ""588patent can be formed with a generally flat hem provided that the sheet metal is still soft after forming and the panel has been suitably formed with a thinned hemline. But there remains a need for the capability of forming a tighter pinch-type hem in SPF aluminum body panels and it is an object of this invention to provide such a method. Furthermore, there also remains a need for the capability of forming a flat hem in non-SPF aluminum body panels and it is a further object of this invention to provide such a method.
This invention provides a method for stretch forming aluminum alloy sheet stock into a body panel or the like, having a flange that can be bent around the edge of an assembled inner panel in a pinched hem. In a pinched hem the outer panel flange is bent with flat portions on each side of the edge of the inner panel and further creased in a fold outboard of the inner panel edge that is thinner than the three metal thickness stack-up of the hem near the inner panel at its edge. A hem that is pinched in this manner provides a tight grip on the inner panel. It is also a very attractive hem for automotive body panels. Such hems have not been attainable in aluminum vehicle panels without a special metal softening heat treatment subsequent to the stamping or stretch forming of the panel. The practice of the invention is particularly useful in the forming of superplastic formable aluminum alloy sheet material but it is not limited to the hemming of SPF aluminum alloys.
The practice of the invention can be illustrated using AA5083. This alloy has a typical composition, by weight, of 4% to 5% magnesium, 0.3 to 1% manganese, a maximum of 0.25% chromium, about 0.1% copper, up to about 0.3% iron, up to about 0.2% silicon, and the balance substantially all aluminum. Generally, a cast alloy ingot is first hot and then cold rolled to a thickness from about one to four millimeters. In SPF AA5083 alloys the microstructure is characterized by a principal phase of a solid solution of magnesium in aluminum with well distributed, finely dispersed particles of intermetallic compounds containing the minor alloying constituents, such as Al6Mn. At the time of superplastic forming, the grain size is less than about ten to fifteen micrometers while the dispersed particle size is less than about two micrometers.
The magnesium containing aluminum alloy sheet stock is heated to a suitable temperature in the range of about 400xc2x0 C. to 510xc2x0 C. (750xc2x0 F. to 950xc2x0 F.) for stretch forming over a suitable tool defining the back of the panel to be formed. Gas pressure is applied to the front of the panel such as is described in the ""588 patent.
In accordance with the invention, hemming flanges are formed at suitable edge locations as the sheet metal blank is progressively stretched into the shape of the panel over a period of a few minutes. The flange portions are progressively stretched and partially bent around a radius portion in the forming tool. The radius is suitably no more than about four times the thickness of the blank material so that sheet metal is selectively stretched and thinned into a hemline just past the bend line of the flange. Preferably the thinned hemline portion is reduced in thickness to about 50 to 90 percent of the thickness of the adjacent flange portion of the newly formed panel.
It is found that the bending of the flange and the formation of the thinned hemline under the pressure of a working fluid does not so work harden the flange that it cannot be subjected to a hemming operation. When the forming is done at an elevated temperature, such as a SPF temperature, the flange portion of the formed panel remains effectively annealed. After cooling and assembly with an inner panel, such thinned flange portions can be completely folded around the end or edge of the inner panel in a hem that is tighter than that of a flat hem. The hem flange can be creased at its edge so that the inside fold is thinner than the inner panel edge. As will be shown, the metal of the thinned flange can also be folded stepwise against the edge of the inner panel so that the hemmed metal lies flat close against the inner panel edge as well as flat against its sheet surfaces. Heretofore, such tight hems have been unattainable without cracking or breaking the folded aluminum sheet.
It is found that the invention is applicable generally to aluminum alloys, such as those of the AA5xxx and AA6xxx series, that are formed with a working fluid, such as a gas or water, with formed-in flanges. The formed flanges have a distinct thinning at the hemline. The forming process may be done at room temperature but more likely it will be done at an elevated temperature.
Other objects and advantages of the invention will become more apparent from a detailed description of a preferred embodiment, which follows.